1. Field of the Invention
The present invention relates to an X-ray mask structure for use in lithography processes in fabrication of large scale integrated circuits (LSI) or micromachines, in which fine patterns are printed by means of X-ray exposure. The present invention also relates to a method for producing the X-ray mask structure and to an exposure method using the X-ray mask structure. Furthermore, the present invention relates to a device fabricated by using the X-ray mask structure.
2. Description of the Related Art
As a result of the very rapid advancement of the technology of very large scale integrated circuits, 4M-DRAMs are now in mass production and 16M-DRAMs or further 64M-DRAMs will be in mass production in the near future. As the technology advances, smaller feature sizes such as 0.5 .mu.m or even 0.25 .mu.m are required in device fabrication. In device fabrication processes, patterns on a mask structure are transferred to a semiconductor substrate by use of near ultraviolet light or deep ultraviolet light. However, the minimum feature size has already nearly reached the ultimate resolution limit that is attainable by using light of such wavelengths. Furthermore, as the device size becomes smaller, an inevitable reduction occurs in the depth of focus. In view of the above, X-ray lithography technology is generally expected to resolve all the above problems at the same time.
In general, a mask structure for use in X-ray exposure comprises patterns of an X-ray absorber formed on an X-ray transmissive membrane on a supporting frame. Among the factors required in X-ray technology using such a structure of an X-ray mask, the most important one is good uniformity in X-ray exposure intensity over an exposed area of a material such as a resist to which patterns are to be transferred. That is, the line width of the patterns transferred to the material to which the patterns are expected to be transferred varies according to the X-ray exposure intensity. When a positive resist is used as a material to which the pattern is to be transferred, the line width of transferred patterns decreases as the intensity of exposing X-rays increases. In contrast, when a negative resist is used, the line width of transferred patterns increases as the intensity of exposing X-rays increases. The variation in intensity of exposing X-rays occurs according to the variation in the thickness of the X-ray transmissive membrane in a fashion as described by the following equation (1): EQU I=Io.multidot.exp(-.mu..multidot.dm) (1)
where I is the intensity of the X-rays after transmitting through the X-ray transmissive membrane, Io is the intensity of incident X-rays, .mu. is the linear absorption coefficient of the X-ray transmissive membrane with respect to the exposing X-rays, and dm is the thickness of the X-ray transmissive membrane.
A conventional X-ray mask structure is not necessarily uniform in thickness of its X-ray transmissive membrane, but there exists a thickness distribution. Therefore, for the reason described above, nonuniformity occurs in the intensity of the X-ray exposure across the area to be exposed at the material to which the pattern is to be transferred. Thus, a very serious problem occurs in that because the designed device line width (that is, the desired line width to be transferred) cannot be reproduced with high fidelity, the high potential abilities of X-ray lithography described above cannot be effectively used to achieve the high integration of devices.